A chemical deposition method has been employed to synthesize BaMgF thin films. Crystal structures of the grown thin films were found to be sensitive to thermal treatment conditions. Only the thin films heated at 550°C for 1 to 2 hours exhibited BaMgF-type structure. Surface composition analysis indicated that anions in the thin films contained not only fluorine ions, but oxygen ions as well. The virtual composition of the thin films is thus BaMgFO instead of BaMgF. The phase transformation and the crystal structures of BaMgFO thin films at different synthesized temperatures were investigated. A new cubic structure of BaMgFO was identified. © 2001 Taylor & Francis.